These experiments will study the kinetics of crossbridge interaction in whole muscle by direct chemical methods. The amount and rate of PCr splitting and the efficiency of chemomechanical coupling will be measured by rapid freezing techniques as a function of shortening velocity, work rate and filament overlap. Thus, direct chemical information will be built into and provide tests of hypotheses of the contractile mechanism, and provide a basis for interpreting heat data. Initial and recovery energy metabolism are closely related; in the steady state the rate of the latter determines the power output of the former. Initial and recovery energy metabolism, maximal rates and controls thereof will be quantitated using methods we developed to study single contractions of isolated muscles. In frog sartorius at 0 degree there was a constant relation between initial PCr hydrolysis and recovery 02 consumption (delta 02); maximal rates of delta 02 appeared to be substrate, not ADP limited, and the apparent P/O ratio was 2, not 3. I will test the generality of these relations, their mechanism and regulation, in rat (red and pale muscles) and frog muscles by measuring the rate of aerobic (delta 02) and anaerobic (delta lactate) recovery metabolism, delta PCr/delta 02 and delta PCr/lactate ratios. Mammalian muscles have a large repertoire of metabolic features and these experiments will exploit the differing strategies of energy metabolism to elucidate control mechanisms. The possible energy cost to maintain sensitive control of metabolic pathways is being investigated.